1. Field of the Invention
The present invention relates, generally, to methods of forming minute structures, for example, nano- or micro-scale structures, and the structures so formed. In particular, aspects of the invention include methods of forming minute structures by selectively exposing photo resists of varying sensitivity to varying dosages of radiation, and the structures so formed.
2. Description of Related Art
The fabrication of microscopic structures, for example, in the nanometer or micrometer size range has attracted increased attention. The fabrication of such structures may typically take the form of “top down” methods, for example, lithographic methods, or “bottom up” methods, for example, colloidal or DNA-type self-assembly. Regardless of the method of assembly, prior art methods of assembly have been limited in their success, for example, prior art methods provided limited fabrication flexibility and were generally not scalable.
One area showing increased promise is the top down method of forming structures employing lithographic methods, for example, employing photo resist materials.
As known in the art, photo resists typically comprise a solution of a liquid solvent and polymer solute. The polymer solute typically comprises polymer chains that are sensitive to radiation, for example, to electromagnetic radiation or electron beam radiation or any type of particle beam radiation. Typically, the polymer solute chains will either combine or “cross link” when exposed to radiation, for example, forming longer, less soluble polymer chains, or cleave or “scission” when exposed to radiation, for example, resulting in shorter, more soluble polymer chains. After reaction with radiation, the liquid solute can be dissolved in an appropriate solvent, for example, an alcohol, removing the scissioned polymer chains or leaving the cross-linked polymer chains to form thin films of polymer on, for example, substrates.
As also known in the art, photo resists are typically available in two “tones”: a “positive-tone” and a “negative-tone.” Positive-tone photo resists comprise solvents having solutes comprising polymer chains that when exposed to certain types of radiation scission and can be dissolved when exposed to an appropriate solvent. In contrast, negative-tone photo resists comprise solvents having solutes comprising polymer chains that when exposed to certain types of radiation cross-link and are not dissolved when exposed to an appropriate solvent.
Both positive-tone and negative-tone photo resists are affected by radiation at varying levels, or dosages, of exposure to the radiation. The polymer solute in a positive-tone photo resist will begin to scission at or about a specific dosage and the polymer solute in a negative-tone photo resist will cross link at or about a specific dosage. These threshold dosages, where reaction of the polymer chains initiates, are referred to as the “base dose” or the “sensitivity” of the photo resist. As known in the art, these base dosages or sensitivities are defined in terms of the unit micro Coulombs per square centimeter (μC/cm2). The dosage provided by radiation or charged particle source, for example, an E-beam writer, typically delivers a dose of radiation or charged particles per unit area per second (for example, Amps/cm2=coulombs/sec/cm2=coulombs/cm2/sec). Therefore, to provide the desired photo resist dosage, the source at the given rate of delivery is provided over a time period (for example, seconds) to provide the desired dose of μC/cm2 to the photo resist (that is, rate×time=μC/cm2/sec×sec=charge in μC/cm2).
The sensitivity of photo resists may vary from single digit μC/cm2, a very high sensitivity, to about 10,000 μC/cm2, a very low sensitivity. However, the typical range of sensitivity of photo resists is from about 100 μC/cm2/ to about 1000 μC/cm2. For example, the negative-tone photo resist hydrogen silsesquioxane (HSQ) has a sensitivity of about 500 μC/cm2 (for example, a lower sensitivity) while the negative-tone photo resist polyvinyl phenol (for example, NEB-31) has a sensitivity of about 30 μC/cm2 (for example, a higher sensitivity). Also, the positive-tone photo resist methyl methacrylate-methacrylic acid (MA-MAA) copolymer has a sensitivity of about 175 μC/cm2 (for example, a higher sensitivity), while the positive-tone photo resist poly(methyl methacrylate) (PMMA) has a sensitivity at about 650 μC/cm2 (for example, a lower sensitivity). These sensitivities are based on an electron beam energy of 100 kV. Different beam energies or charged particles will exhibit different sensitivities.
Aspects of the present invention provide methods of producing structures, for example, nano- or micro-scale structures, by tailoring the assembly of layers of photo resist of varying sensitivity and exposing the layers to dosages of radiation or charge particles to selectively cross-link or scission portions of the layers of photo resist in the photo resist stack and then developing the photo resist stack with an appropriate solvent to form three-dimensional complex structures.